1. Field of the Invention
This invention relates generally to the field of receiving antennas for miniature communication receivers, and more particularly to an integral antenna incorporating a loss cancellation element for use in miniature communication receivers utilizing multi-board construction techniques.
2. Description of the Prior Art
Miniature communications receivers have utilized numerous antenna configurations to achieve reasonable sensitivity levels. Single-turn and multi-turn loop, antennas with and without ferrite cores have generally been utilized to provide integral antennas within the communication receiver housing. However, as the size of communication receivers, such as paging receivers, have diminished, the ability to maintain receiver sensitivities with conventional receiving antenna configurations have become more difficult, due to both general reductions in the size of the receiving antenna, and due to increased interaction of the receiving antenna with other circuit elements within the receiver. As the size of the receiver housing has decreased, the relative size of the receiving antenna had to increase to overcome at least some of the sensitivity loss due to the smaller receiver size, in particular the cross sectional area in the plane of the antenna. The relative increase in the size of the antenna has correspondingly increased the probability of interaction of the antenna with other elements of the receiver and decoder. In many cases the PC boards comprising the decoder/receiver are so densely populated that there is no room to move the elements away from the antenna area. The interaction with the antenna was especially pronounced when the antenna configuration completely enclosed the receiver and decoder electronics. As a result, the resultant receiver sensitivity was often found to be significantly less than other antenna configurations which were able to minimize the interactions by positioning the antenna apart from the circuit elements.
Various approaches have been taken to minimize or eliminate the interactions between the antenna and the receiver and decoder elements. In U.S. Pat. No. 4,862,181 to Ponce De Leon et al, entitled "Miniature Integral Antenna-Radio Apparatus", a single turn tuned floating loop was utilized to minimize the interactions. The floating loop was coupled to the receiver input using a coupler element which prevented the formation of undesired signal loss paths between the antenna and the receiver and provided impedance matching to the receiver input. U.S. Pat. No. 4,814,776, to Caci et al, entitled "Optimally Grounded Small Loop Antenna" describes an antenna configuration which completely enclosed the receiver, and described an isolation network which canceled the interaction of the antenna and the receiver by providing an optimum ground to the antenna. U.S. Pat. No. 4,491,978 to Nagata et al entitled "Portable Radio Receiver with High Antenna Gain" described an antenna configuration which substantially enclosed the receiver. The interaction of the antenna with the receiver elements was minimized by providing high impedance elements in series with at least the power supply and grounding lines at some point between the power supply and the RF-IF converter circuits. A high impedance element in series with the signal path could be optionally provided at some point downstream of the RF-IF converter circuits.
While each of the antenna configurations described above have successfully resolved problems of interaction between the antenna and the receiver and decoder elements in a receiver design having a single board configuration, significant problems still exist when the portable communication receiver configuration includes multiple boards such as shown in the sectional and planar views of FIGS. 1B through 1D, and depicted by the electrical block diagram of FIG. 1A. Referring to FIG. IA, in the multi-board configuration, the receiver components are generally located on a receiver board 10 which also includes those elements of the antenna 14. Other functions were generally located on a separate decoder board 16, which included such functions as the decoder, a display driver and display, alerting circuitry, a code plug or address and function memory, and other decoder and receiver control functions, such as battery saver controls and user operated switches. The multi-board configuration, as shown in FIG. 1A, allows multiple decoder boards, such as a decoder board for decoding the Golay Sequential Code (GSC) signaling format, or a decoder board for decoding the POCSAG signaling format, to be used with a common receiver board, such as a receiver board for the VHF, UHF or 900 MHz operating frequencies. Because of the proximity of the antenna, which is located on the receiver board 10, to any number of the components which are located on the decoder board 16, interactions which are depicted by reference numeral 12, can occur between the antenna 14 and those decoder board components in close proximity with the antenna, resulting in reduced receiver sensitivity. Significant sensitivity differences also resulted when switching from one type of decoder board to another.
Referring to FIG. IB, the interactions (shown by reference numeral 12) between the antenna 14 located on the receiver board 10 and arbitrary decoder circuit elements 18a, 18b and 18c are shown. For purposes of clarity, FIGS. 1B and 1D are drawn with the receiver board 10 spaced apart from the decoder board 16. In actual usage, when the receiver board and the decoder board are coupled together and positioned within the communication receiver housing, the antenna 14 of FIG. 1B would normally be in very close proximity to the decoder circuit elements, generally being separated only by an insulating spacer 22. While the decoder circuit elements identified in FIG. 1B represent physical components, such as resistors, capacitors, inductors and other electrical components, it will be appreciated that the interactions 12 can also occur due to the close proximity of the circuit traces 20a, 20b and 20c as shown in FIG. 1C which lie in close proximity to, or may actually traverse beneath the antenna element 14. It will be appreciated that the number of sites at which interactions 12 occur are functions of the proximity of the decoder circuit elements to the antenna element 14. The level of the interaction, and the resultant loss in sensitivity due to such interactions has been found to be a function of whether the interaction sites represent coupling from low impedance or high impedance circuits relative to the impedance of the antenna at the operating frequency of the receiver. It is desirable to maintain low impedance circuits sufficiently far from the antenna element 14, so as to minimize the interaction of the antenna with the decoder circuit elements. However, as previously stated above, as the size of the communication receivers are diminished, there becomes less flexibility in providing the optimum separation of the antenna and decoder circuit elements.